1. Field of the Invention
The invention relates to apparatus for measuring magnetic field information, and may be used in particular for determining components of magnetic field gradient. The system may be used to detect magnetic field gradients of the order of 100 fT mxe2x88x921 from a moving platform in the background of the earth""s magnetic field (approximately 70 xcexcT).
2. Discussion of Prior Art
SQUID (superconducting quantum interference devices) magnetometers are extremely sensitive devices which can measure vector components of magnetic field as small as 10xe2x88x929 times the ambient field of the earth. Measurement of fields near to the threshold of sensitivity in the presence of the earth""s field present many difficulties. Geomagnetic noise and man-made noise, for example, are always dominant. Furthermore, unless the SQUIDs are rigidly mounted, even minute motions in the earth""s field will be reflected as gross changes in output and therefore it is often more useful to sense the gradient in a magnetic field, rather than the field itself.
The gradient of a magnetic field may be measured by using an intrinsic SQUID gradiometer. An intrinsic SQUID gradiometer has sensing coils made up of two loops connected with opposite polarity. In such a configuration the sensing coils must be highly balanced and aligned and small fractional changes in the effective size or orientation of each coil produce output signals from uniform fields which are indistinguishable from real gradients. Balancing is usually achieved by adjustment in special calibration rigs under laboratory conditions and is an expensive and time consuming process. Furthermore, as a large background field is incident on the sensor, currents are induced in the structure which give rise to 1/f noise and can give rise to hysteresis problems.
The gradient signal is larger for coils separated by a longer baseline, the useable baseline being limited by the tolerable inductance in the connections to the coils. This restriction on baseline can be removed by replacing the gradiometer sensor by a pair of magnetometers, where outputs are subtracted to form a configured gradiometer. Such a configuration requires great stability and linearity in both time and frequency domains. Furthermore, as well as the difficulties in balancing, each sensor requires a very large dynamic range (better than 1 part in 109) if it is to be operable on a moving platform, as is often required. In addition, there is still the problem that the field is incident on the structure.
In U.S. Pat. No. 5 122 744, an aligned three sensor configured gradiometer (Three SQUID Gradiometer, TSG) is described in which a central sensor is used to feedback that component of the earth""s field to coils surrounding each of the three sensors. The outputs of the outer pair of sensors are subtracted and this difference gives a measure of the required gradient if the feedback field is uniform. In this configuration, the dynamic range is considerably improved and one component of the earth""s field is not directly incident upon the sensors.
The same technique has been extended to gradiometers based on fluxgate magnetometers rather than SQUID magnetometers [R. H. Koch et al. xe2x80x9cRoom temperature three sensor magnetic field gradiometerxe2x80x9d Review of Scientific Instruments, Jan. 1996, AIP, USA, vol. 67, No. 1, pages 230-235].
However, the problems of stability, non-linearity, uniformity of feedback and the need to calibrate and fix the balance are not overcome. It is the difficulty of accurately subtracting the sensor outputs which give rise to many of the problems.
Also relevant to the present invention is the processing technique used to compensate for motion noise in extremely low frequency submarine receiving antennas [R. J. Dinger and J. R. Davis, Proc. IEEE, vol.64, No. 10, Oct. 1976].
An additional technique for improving the performance of SQUID sensors, based on adaptive positive feedback, is known from U.S. Pat. No. 5,488,295.
For the purposes of this specification, the term magnetometer shall be taken to refer to a device for measuring the magnetic field component in a particular direction and the term gradiometer shall be taken to refer to a device for measuring magnetic field gradient components. A total field magnetometer shall be taken to refer a device for measuring the total magnetic field i.e. the square root of the sum of the squares of the magnetic field components in three orthogonal directions.
According to the present invention, a system for measuring one or more magnetic field gradient component of a magnetic field comprises;
(i) at least two magnetic sensors for sensing a magnetic field, wherein each sensor generates a sensor output, said sensor outputs having an associated total energy, E, and wherein at least two of the sensors are arranged to sense the magnetic field in substantially the same direction and
(ii) means for performing adaptive signal processing of the sensor outputs such that the system is adaptively balanced, whereby said means generate one or more magnetic field gradient components, characterised in that
(iii) the system incorporates global feedback means for providing a substantially uniform magnetic field at the two or magnetic sensors, and
(iv) the adaptive signal processing means comprise means for minimising the total energy, E, of the sensor outputs subject to a constraint, whereby the constraint determines which of one or more magnetic field gradient components is generated.
The invention provides the advantage that it eliminates the need to calibrate and fix the balance under controlled conditions and to maintain that calibration for long periods. Furthermore, the requirements on the mechanical rigidity and stability required for low noise operation are considerably relaxed. The advantages of the known configured systems are maintained.
In a preferred embodiment, the means for minimising the total energy, E, of the sensor outputs may also generate a total magnetic field measurement, whereby the constraint determines which of one or more magnetic field gradient components or a total magnetic field measurement is generated.
In a further preferred embodiment, the gradiometer may comprise a computer on which an adaptive signal processing algorithm (ASPA) is loaded.
The gradiometer global feedback means may also be arranged to provide a substantially uniform magnetic field gradient at the two or more magnetic sensors. The global feedback means may comprise at least one global feedback coil set. For example, each set may comprise two or more Helmholtz coils.
The gradiometer may also comprise;
means for generating at least one difference signal between two sensor outputs, wherein said sensor outputs each correspond to a magnetic field in substantially the same direction, and analogue to digital conversion means for converting the one or more difference signal and the two or more sensor outputs into equivalent digital data.
At least one of the magnetic sensors may be one of a fluxgate, a Hall probe, a magneto-resistive sensor or a superconducting quantum interference device (SQUID) magnetometer. Alternatively, at least one of the magnetic sensors may be a gradiometer.
If a SQUID magnetometer is included in the gradiometer, the gradiometer also includes cooling means for reducing the temperature of the SQUID magnetometer. Each SQUID magnetometer may have associated local feedback means for maintaining a substantially constant state of magnetic flux in the respective SQUID magnetometer.
In one embodiment of the invention, the gradiometer comprises;
at least four magnetic sensors for sensing a magnetic field, wherein three of the sensors are arranged such that they sense the magnetic field in three substantially orthogonal directions and wherein at least two of said sensors are arranged such that they sense the magnetic field in substantially the same direction.
In a further preferred embodiment, the gradiometer may comprise at least eight magnetic sensors. For example, the eight or more magnetic sensors may be arranged at the vertices of a tetrahedron structure. In this configuration, the three global feedback coil sets may be oriented in three substantially orthogonal directions, for generating a substantially uniform magnetic field at each of the eight or more magnetic sensors.
In another embodiment, the gradiometer may comprise at least three magnetic sensors, wherein at least three of the sensors are oriented in substantially the same direction and whereby the means for performing adaptive signal processing may generate a magnetic field gradient component of at least second order.
In another embodiment of the invention, the system may be arranged to provide a biomagnetic sensing system. The at least one global feedback coil set may be arranged to surround a subject, for example a human subject, generating a magnetic field to be measured wherein the subject may be in close proximity to the magnetic sensors. This system provides an advantage over conventional biomagnetic sensing systems in that the large magnetically shielded room required in conventional systems is no longer needed.
According to another aspect of the invention, a method for measuring at least one magnetic field gradient component using a gradiometer comprises the steps of;
(i) sensing a magnetic field component at two or more positions using two or more magnetic sensors, wherein at least two of the magnetic field components are sensed in substantially the same direction,
(ii) generating two or more output signals, having an associated total energy, E, corresponding to said magnetic field components,
(iii) providing global feedback in the form of a substantially uniform magnetic field at the two or more magnetic sensors,
(iv) performing adaptive signal processing of the output signals and minimising the total energy, E, of the output signals subject to a constraint such that the gradiometer is adaptively balanced,
(v) constraining the minimisation of the total energy, E, such that a magnetic field gradient component is generated, and
(vi) generating at least one magnetic field gradient component measurement.
In one embodiment of this aspect of the invention, the method may comprise the steps of;
(i) minimising the total energy, E, of the output signals subject to a constraint such that the gradiometer is adaptively balanced, and
(ii) constraining the minimisation of the total energy, E, such that a total magnetic field measurement is generated.
In an alternative embodiment of this aspect of the invention, the method may comprise the step of providing a substantially uniform magnetic field gradient at the two or more magnetic sensors. The method may be used for measuring magnetic field components in a human subject in close proximity with the two or more magnetic sensors.
In another embodiment of this aspect of the invention, the method may comprise the steps of;
(i) sensing the magnetic field component at three or more positions, wherein at least three of said magnetic field components are sensed in substantially the same direction,
(ii) minimising the total energy, E, of the output signals subject to a constraint such that the gradiometer is adaptively balanced, and
(iii) constraining the minimisation of the total energy, E, such that a magnetic field gradient component of at least second order is generated.
The system utilises an adaptive signal processing technique to achieve high levels of balance. The application of this technique to magnetic detection with gradiometer systems is unknown. The employment of adaptive signal processing is essential to allow the system to utilise a scheme of global (or overall) magnetic field feedback. For a system from a moving platform, this provides sufficient dynamic range for the system to operate in the earth""s field whilst maintaining high levels of balance. Furthermore, it also provides the advantage that it prohibits the ambient external field impinging directly on the sensors. Furthermore, it relaxes the mechanical rigidity constraints for the system.